Chemical vapor deposition (CVD) and atomic layer deposition (ALD) have been applied as main deposition techniques for depositing thin films for semiconductor devices because it enables the achievement of conformal films (metal, oxide, nitride . . . etc) through fine tuning of parameters during the process. The film growth is mainly controlled by the chemical reaction of metal-organic compounds (precursors), so the development of optimum precursors is essential under prediction of its property and reaction process. Precursors have been developed to reach required properties based on its specific application to certain types of film.
Titanium and aluminum based films find many different applications useful for the fabrication of nano-devices, such as semi-conductor devices, Microelectrochemical systems (MEMS) devices, or photovoltaic devices, but also in manufacturing of tools or machines. In semiconductor applications, the use of TiAl(N) (titanium-aluminum-nitride) layers is considered for its high oxidation resistance, as well as the low resistivity of such materials. TiAl or TiAlON could replace advantageously TiN films for NMOS metal gate electrode. Kesapragada et al., High-k/Metal Gate Stacks in Gate First and Replacement Gate Schemes, ASMC 2010, pp. 256-259; Miyoshi et al., Titanium-aluminum Oxynitride (TAON) as High-k Gate Dielectric for Sub-32 nm CMOS Technology, Microelectronic Engineering 87 (2010) 267-270. Used as a diffusion barrier between copper and low-k, TiAlN alloy also proved to exhibit very good blocking properties up to 1000° C. Lee et al., Study of Diffusion Barrier Properties of Ternary Alloy (TixAlyNz) in Cu/TixAlyNz/SiO2/Si Thin Film Structure, Materials Science in Semiconductor Processing 3 (2000) 191-194. Kawata et al. evaluated the structural, mechanical, and chemical properties of Ti0.58Al0.42N (upper)/TiN (lower films), revealing high oxidation resistance, a low friction coefficient, and high wear resistance. Characterization of (Ti,Al)N Films Deposited by Pulsed D.C. Plasma-Enhanced Chemical Vapor Deposition, Thin Solid Films 386, 2001, 271-275.
Deposition of titanium-aluminum (TiAl) alloy may be used as bimorph actuators in MEMS devices. Qu et al., Characterization of TiAl Alloy Films for Potential Application in MEMS Bimorph Actuators, Materials Science in Semiconductor Processing 5 (2002) 35-38. The advantage of TiAl over aluminum (Al) for this application stems in the instability and high stress gradient of Al.
TiAl(N) films are also of interest for hard coating applications in the tooling or machining manufacturing area because of the high wear and oxidation resistance properties. Chokwatvikul et al., Effect of Nitrogen Partial Pressure on Characteristic and Mechanical Properties of Hard Coating TiAlN Film, Journal of Metals, Materials and Minerals, Vol. 21, No. 1, pp. 115-119, 2011. Still in the coating area, but for bio implants applications, it was observed that TiAlN films exhibited superior electrochemical corrosion resistance compared to TiON and TiN. Subramanian et al., A Comparative Study of Titanium Nitride (TiN), Titanium Oxynitride (TiON), and Titanium Aluminum Nitride (TiAlN), as Surface Coatings for Bio Implants, Surface & Coatings Technology 205 (2011) 5014-5020.
The interest for TiAl-based materials is thus very high. Besides, the need is clearly directed to nano-devices, where depositions of films of a few Angstroms thickness in very constraining geometries, including deep trenches or 3D geometries, are needed. Deposition methods that allow deposition and control at the atom level are desired. Techniques using vapors of organometallic molecules, also called precursors, in Chemical Vapor Deposition (CVD) and Atomic Layer Deposition (ALD) modes are considered to be effective methods to fulfill these needs.
However, available precursors suitable for such vapor phase deposition are relatively scarce. Two different approaches could be considered to deposit TiAl-based films: reaction of two distinctive molecules of titanium (Ti) and aluminum (Al) or usage of a single molecule, intrinsically containing Ti and Al.
The first approach has the advantage of providing better individual control of the deposition parameters, but some issues such as the non-overlapping of two process windows based on both precursors' process windows may occur. Such kind of issues could be solved by using a single source molecule, a bi-metallic precursor containing Ti and Al.
The issue with such molecules is the very limited availability of molecules possessing qualities to be used in gas phase depositions. Namely, the scarcity, proven by the lack of the corresponding literatures, of bi-metallic precursors that possess all or most of the following properties: liquid, sufficiently volatile, stable at storage and delivery conditions, reactive to heat or to other reactants (such as hydrogen (H2), ammonia (NH3), hydrazine (N2H4), etc), among others.
Ideally the molecules would not be composed of carbon, or would have as little metal to carbon bonds as possible. The way the ligand is linked to the metal atom is also of high importance, as the lower the bond energy between the metal and ligand, the easier the bond will be broken, potentially leading to impurity-free film, as long as the ligand does not decompose itself during the deposition process.
The number of molecules answering to this criterion is limited. Tris(tetrahydroborate)titanium (TiBH4)3 would be of interest but is reported to be unsuitable for CVD applications because it decomposes thermally at −50° C. Girolami et al., Low Temperature MOCVD Routes to Thin Films from Transition Metal Precursors, Mat. Res. Soc. Symp. Proc. Vol. 168, 1990. However, the stability of this molecule increases by the addition of donor molecules. The 1,2-dimethoxyethane adduct Ti(BH4)3(dme) is reported to be stable at room temperature and sublimes readily. Chemical vapor deposition using this precursor gives thin films of TiB2 at temperatures as low as 250° C. Id.
Some other molecules are also described, such as Cp2Ti(AlH4). Abstract of Koordinatsionnaya Khimiya (1985), 11 (3), 339-45.
In conclusion, a need remains for molecules with satisfactory properties that allow the deposition of good quality TiAl-based films using single source bi-metallic TiAl precursors.